1. Field of the Invention
The present invention is in the field of read-only memories and in particular in semiconductor read-only memories wherein each memory cell has the capability of storing more than one bit of binary information.
2. Prior Art
One of the largest components of a digital computer system is the memory banks. Although substantial improvements have been made in computer memories, the size, speed and storage capacity of memories tend to limit computer systems, particularly mini- and micro-computer systems which are typically required for small-scale and on-site applications.
Present technology has continued to improve the storage capacity of semiconductor memories principally by reducing the device or memory cell size on the semiconductor chip. However, the device and memory cell sizes have decreased on semiconductor chips to the point of the practical and physical limitations of micro-photolithographic techniques. Therefore what is needed is a semiconductor memory which can be designed within the drafting rules and practical limitations of micro-photolithography and whereby digital information density can be substantially increased in a semiconductor memory chip.